I have used a MMi3, and played against two guys that used them. I found it too bottom heavy, personally.
One of the guys I played against got out third ball- got a short ball (we had a man back on the hook), he had to take it on, and was caught.
Haven't seen/used a CoR3 so can't comment on them, but personally don't see the point of them over a normal shorter blade/longer handle bat. Surely an Uzi or similar will have more whip in the handle which will more than outweigh the (small) benefit of not having a splice. Personally I don't use the splice that much, and the odd time I do (a rising bouncer), I'm quite relieved that it is a dead spot on the bat and drops at my feet...